The invention relates to a power supply device for a variable rotation speed drive which is arranged on the seabed.
Variable rotation speed drives, consisting of an electric motor and a pump or a compressor, are supplied by means of a power supply device such as this, when on the seabed, with power from an electrical power supply system on land. The distance between the feeder on land and the drive on the seabed may be several hundred kilometers, at sea depths of several kilometers.
Variable rotation speed drives for underwater applications, also referred to as subsea applications, are used, for example, for delivering oil and gas from the seabed. These variable rotation speed drives are, as is known, supplied with power from an electrical power supply system by means of a voltage intermediate-circuit converter.
The publication entitled “Stromrichterschaltungen für Mittelspannung und deren Leistungshalbleiter für den Einsatz in lndustriestromrichtern” [Power converter circuits for medium voltage and their power semiconductors for use in industrial power converters] by Max Beuermann, Marc Hiller and Dr. Rainer Sommer, printed in the Proceedings of the ETG Conference “Bauelemente der Leistungselektronik und ihre Anwendung” [Power electronics components and their use], Bad Nauheim, 2006, discloses a plurality of converter topologies for implementation of medium-voltage converters. These converter topologies include 3-level neutral-point clamped (3L-NPC) converters with a 12-pulse diode feed, a 4-level flying capacitor (4L-FC) with a 12-pulse diode feed, series-connected H-bridge cell converters with 2-level H-bridges per cell (SC-HB(2L)) and a series-connected H-bridge cell converter with one 3-level H-bridge per cell (SC-HB(3L)).
The publication “Modulares Stromrichterkonzept für Netzkupplungsanwendung bei hohen Spannungen” [Modular converter concept for power supply system coupling use at high voltages], by Rainer Marquardt, Anton Lesnicar and Jürgen Hildinger, printed in the Proceedings of the ETG Conference 2002, discloses a converter with a power converter on the power supply system side and on the load side, which are electrically conductively connected to one another on the DC voltage side, wherein a modular multipin power converter, also referred to as a modular multilevel converter (M2C), in each case being used as the power converter. A voltage intermediate-circuit converter such as this with a power converter on the power supply system side and on the load side, based on M2C topology, no longer has a voltage intermediate circuit, formed from intermediate-circuit capacitors, in comparison to the already described voltage intermediate-circuit converters. Each valve branch in each phase module of the converter based on M2C topology has at least one two-pole subsystem. The steps in a phase output voltage are governed by the number of subsystems used in each valve branch.
The object of a power supply device for subsea applications is to supply a motor, which is located on the seabed, for a variable rotation speed drive, with a polyphase voltage system with a variable voltage and frequency. There are various fundamental embodiments in this case:
FIG. 1 schematically illustrates a first known variant of a power supply device for subsea applications. In this FIG. 1, 2 denotes a voltage intermediate-circuit converter, 4 an electric motor for a drive, 6 a power converter transformer and 8 a feeding power supply system. The voltage intermediate-circuit converter 2 has a power converter 10 and 12 on the power supply system side and on the load side, which are electrically connected to one another on the DC voltage side by means of a DC voltage intermediate circuit, which is not shown explicitly, for clarity reasons. The power converter on the load side, which is preferably a self-commutated pulse-controlled power converter, is linked to the motor 4 by means of a three-phase alternating-current cable 14. In addition, this converter 2 has signal electronics 16, which can be connected on the input side by means of a data line 18 to connections of the electric motor 4, and this data cable 18 is therefore shown by means of a dashed line, and is connected on the output side to control connections of the self-commutated pulse-controlled power converter 12. A transformer with two secondary windings 20 and 22 is provided as a power converter transformer 6, of which windings the secondary winding 20 is connected in delta, and the secondary winding 22 is connected in star. Since the primary winding 24 is likewise connected in star, only the secondary winding 20 has a phase-shift angle with respect to the primary winding 24 of 30° electrical. The primary winding 24 is electrically conductively connected to the feeding power supply system 8, in particular to a feed point 26. A 12-pulse diode feed is provided as the power converter 10 on the power supply system side. This means that this diode feed 10 has two three-phase diode bridges, which are electrically connected in series on the DC voltage side. The 12-pulse embodiment of the diode feed 10 results in the current overshoots in the feeding power supply system 8 being small. This power supply device for subsea applications is arranged on land, or on a platform at sea. The transition from the land or platform to the sea is indicated by the wavy lines 30. Therefore, only the drive, consisting of the motor 4 and a pump and/or a compressor, is located on the seabed. Of the drive, only the motor 4 is shown in any detail.
Since the capacitive charging power of the alternating-current cable 14 places a major requirement for reactive power on the voltage intermediate-circuit converter 2, there can be only a limited distance between the converter 2 and the motor 4. In addition, this power supply device does not allow a multiple-motor drive. Each motor 4 in a drive must be connected by means of its own alternating-current cable 14 to the voltage intermediate-circuit converter 2.
FIG. 2 shows a further known power supply device for a variable rotation speed drive which is arranged on the seabed. This embodiment differs from the embodiment shown in FIG. 1 in that the alternating-current cable 14 is linked by means of a transformer 32 to outputs of the self-commutated pulse-controlled power converter 12 in the voltage intermediate-circuit converter 2. In addition, this alternating-current cable 14 is connected by means of a second transformer 34 to connections of the electric motor 14 which is arranged on the seabed. The transformer 32 transforms a generated converter voltage to a potential which is higher than the potential of the rated voltage of the electric motor 4. After transmission, this potential is transformed back to the rated potential of the motor. The increased transmission voltage results in reduced resistive power losses. In addition, the alternating-current cable 14 may have a smaller cable cross section, thus allowing a better design of the cable 14. This makes it possible to bridge a greater distance between the converter 2 and the motor 4, in comparison to the embodiment shown in FIG. 1. These advantages are countered by the need for two transformers 32 and 34, and the transformer 34 on the seabed must be encapsulated. When supplying power to a plurality of motors 4 on the seabed, two further transformers 32, 34 must also be provided for each further motor 4, as well as a further cable 14.
In a further variant of the power supply device for a variable rotation speed drive on the seabed, the voltage intermediate-circuit converter 2 is arranged with the power converter transformer 6 on the power supply system side, as shown in FIG. 3, on the seabed in the immediate vicinity of the electric motor 4 for the variable rotation speed drive. A power supply system transformer 36 is provided on land, whose primary is electrically conductively connected to the feeding power supply system 8, in particular to the feed point 26, and whose secondary is electrically conductively connected to the alternating-current cable 14. The use of the power supply system transformer 36 allows the AC voltage to be transmitted to be transformed to a value which is above the potential of the rated voltage of the electric motor 4. This transmission voltage is transformed down again by the power converter transformer 6.
In this power supply device, only the transformer 36 is still located on land or on a platform arranged at sea. The voltage intermediate-circuit converter 2 is now located adjacent to the motor 4 on the seabed, and can be directly linked to the motor 4. This improves the drive performance, although the converter 2 now also has to be encapsulated. In comparison to the variant of the power supply device shown in FIG. 2, this changes nothing with regard to the distance between the feed point 26 and the motor 4.
FIG. 4 schematically illustrates a multiple-motor variant of the power supply device shown in FIG. 4. Each motor for a variable rotation speed drive is coupled to a voltage intermediate-circuit converter 2 by means of a power converter transformer 6 on the power supply system side. On the seabed, the alternating-current cable 14 is linked to an alternating-current busbar 38, to which the plurality of power-converter-fed drives are connected. The alternating-current cable 14 can be provided with a further transformer 40 on the load side on the seabed, whose secondary is linked to the alternating-current busbar 38. Since this further transformer 40 is not absolutely essential, it is illustrated by means of dashed lines. This further transformer 40 results in the AC voltage busbar 38 being at a potential below the potential of the transmission voltage, but above the potential of the rated voltage of the electric motor 4 for a variable rotation speed drive. In this variant as well, the distance between the feed point 26 on land and the drive on the seabed is still restricted, as in the case of the other variants shown in FIGS. 1 to 3. Furthermore, the number of installation parts on the seabed has increased many times. All the installation parts which are arranged on the seabed must be accommodated in an encapsulated form, in particular in each case in a pressure vessel.
The publication “Valhall Re-Development Project, Power from Shore” by Sverre Gilje and Lars Carlsson, printed in “ENERGEX 2006”, discloses a power supply device which connects a platform at sea to a feed point on land. The known “light” version of the high-voltage, direct-current transmission installation is provided as the power supply device. This HVDC light has two self-commutated pulse-controlled power converters, which are connected to one another on the DC voltage side by means of a direct-current cable. Each of these two self-commutated pulse-controlled power converters has an alternating-current filter on the AC voltage side, and a capacitor and a direct-current filter on the DC voltage side. The one self-commutated pulse-controlled power converter is arranged by means of a power supply system transformer at a feed point of a feeding power supply system on land while, in contrast, the second self-commutated pulse-controlled power converter is arranged on a platform at sea. A sea cable with a length of approximately 300 km is provided as the DC voltage cable. No communication is required between these two power converters. All that is required is the value of the DC voltage at both ends of the DC voltage cable. The power converter station on land controls the transmission voltage, and the power converter station on the platform at sea controls the real power. The distance between the feed point and the platform is likewise restricted with this power supply device, as well.
The invention is now based on the object of developing the known power supply device such that the distance between the feed point on land and the drive on the seabed is considerably greater.